Temporary crowns and bridges in synthetic material are an important part of prosthetic treatment. An essential requirement of these materials is that they guarantee a smooth surface finish of the temporary prosthesis, since rough surfaces promote intra-oral plaque adhesion. If in the first place a bacterial film has formed, this can rapidly lead to inflammatory reactions of the periodontium and to gingivitis. A smooth surface also helps to prevent aesthetically problematical discolorations, such as for example those caused by coffee, tea, wine or cigarettes. The use of a tooth lacquer also provides a sheen to the surface of temporary prostheses with an otherwise matt appearance.
Generally surface smoothing of temporary prostheses is generally performed either with the help of polishes and polishing pastes or using unfilled or only very low-filled protective lacquer systems. Many investigations have shown that lacquered temporary prostheses demonstrate lower plaque build-up than polished surfaces. The reason for this is that when conventional polishing and abrasive bodies are used, all products to some extent leave behind working traces which are unavoidable when using mechanical polishing methods. For this reason the use of protective lacquers on these substrates is recommended.
When using a protective and gloss lacquer for coating of prostheses and temporary prostheses, account must be taken of the fact that the cured system after a certain dwell time on the prosthesis or the temporary prosthesis will have a rougher surface than directly after application of the lacquer. Such a situation can arise if the lacquer partly comes away from the synthetic material and leaves behind only “islands” with a gradual transition to residual synthetic material. At these points germ cells can then result in bacterial colonization.
Protective and gloss lacquers are known from the literature.
JP 4-29910 A describes the composition of a dental, colorless and transparent coating material that is claimed to have exceptional abrasion resistance. The lacquer contains a polyfunctional cross-linker, which is preparable through esterification of more than three hydroxyl groups of pentaerythritol or dipentaerthritol with acrylic acid (in particular pentaerythritol hexaacrylate), a volatile methacrylate compound (in particular methyl methacrylate) and an acyl phosphine oxide as the polymerization catalyst.
U.S. Pat. No. 7,081,485 B2 discloses a photocurable composition that can be used as a dental coating material, comprising a multiacrylate compound and a photoinitiator (acyl phosphine oxide), wherein the multiacrylate compound has at least five acrylate functionalities per molecule and the composition contains no methyl methacrylate and whereby the photoiniator is present in a quantity of less than 6 wt. % in the composition. The photoinitiator preferred here is 2,4,6-trimethylbenzoyl diphenyl phosphine oxide. The lacquer is claimed to cure free of any smear layers.
US 2005/0288387 A1 likewise relates to dental coating compositions that are claimed to cure without smear layers. These compositions are claimed to be storable with low odor and comprise a multiacrylate compound, a photoiniator (acyl phosphine oxide) and an alcohol, wherein ethanol is preferably used.
US 2004/0034119 A1 likewise discloses a dental material that is claimed to cure without a smear layer. Here a lacquer is initially applied to the tooth. In a second step a cover layer, for example a sealed, continuous layer of an essential oil, is applied to the surface of the lacquer. The lacquer is thus shielded from the surrounding atmosphere and can then be polymerized without a smear layer. The advantage of this method is claimed to be the fact that the covering layer, depending on the selection of its composition, provides an agreeable taste and or an agreeable smell during handling. Optionally the covering layer can be removed following polymerization.
US 2006/0111465 A1 claims dental polymerizable varnishes that contain a polymerizable, ethylenically unsaturated resin composition as well as a curing agent and a filler, which substantially constitutes a polyhedral oligomeric silsesquioxane.
EP 1 532 958 B1 is aimed at a dental coating material, that contains nanoscale particles with average particle sizes of 1 through 100 nm, that have been surface-modified, at least one polyfunctional methacrylate monomer, having four or a plurality of methacryloyl groups in a molecule and a photopolymerizator.
US 2007/0010597 A1 discloses laser-curable compositions for protecting the dental hard tissue. The compositions are single component and contain curable monomers with at least two polymerizable double bonds in the molecule and benzoyl peroxide as the initiator.
DE 10 2007 048 925 A1 describes coating and lacquering compositions which inter alia can be used for prostheses, lining materials or implants. The compositions described there are polysiloxane-based.
A further dental lacquer is disclosed in DE 10 2008 010 464 A1. In the event that, as detailed above, the lacquer layers over time, for example in abrasively stressed areas of the tooth surface, become permeable with the partial absence of lacquer and exposure of unprotected areas of the tooth surface, then a more intense build-up of bacterial deposits there is likely. For this reason tooth lacquers have been proposed that prevent or make more difficult the adherence of deposits to the lacquer. According to DE 10 2008 010 464 A1 this is claimed to be achieved by using a lacquer that through urethane groups in the dental material contains incorporated perfluorinated radicals. The perfluorinated alkyl groups that are incorporated into the lacquer formulation by reaction of perfluoro alcohols with isocyanate groups serve to render hydrophobic the surface of the cured system. Such hydrophobic surfaces are claimed to make more difficult or prevent the occurrence of deposits.
DE 10 2009 011 537 A1 relates to dental surface coating materials, which following polymerization lead to less discoloration. The lacquer system contains 2,4,6-trimethylbenzoyl diphenyl phosphine oxide as a photoinitiator and dimethacrylate bonds formed by ethylene oxide units, wherein the number of ethylene oxide structure elements is between 9 and 50. The lacquer can also contain a monomer such as dipentaerythrityl hexaacrylate.
DE 10 2009 011 536 A1 reports that the use of a (bis)acyl phosphine oxide compound such as 2,4,6-trimethylbenzoyl diphenyl phosphine oxide in combination with a diketone compound such as camphor quinine in the absence of an amine in a photocurable monomer system has a number of effects, e.g. an improvement in the curability in a broad wavelength range, reduction in the change in color tone before and after curing and an improvement in the thin layer surface curability. As preferred organic matrix monomers, dipentaerythrityl hexaacrylate and dimethacrylate compounds formed by alkylene oxide units are mentioned with the number of alykylene oxide structure elements being between 5 and 50.
DE 10 2008 039 129 A1 relates to transparent, scratch-proof coatings, which are also envisaged for use in dentistry. The coating material comprises a siloxane-containing matrix and functionalized single- and multiple-walled carbon nanotubes dispersed therein.
U.S. Pat. No. 6,617,413 B1 describes coating agents which can be hardened by the addition of isocyanate groups as well as by the radiation-induced addition of activated c-c double covalent bonds. The compounds described there can inter alia be used in dental compounds.
EP 1 307 173 B1 describes a fluoride lacquer and silicon-based fissure sealing material.
A temporary, transparent, dental lacquer composition for coating of tooth surfaces is described in US 2006/0063853 A1. Here the transparency is brought about by the addition of hollow glass balls, wherein the hollow glass balls divert any incoming light back to the light source. This effect is brought about by a total reflection that takes place inside the balls, which always occurs if light enters a hollow glass ball. This phenomenon is used in particular in road markings. In order to be able to create this effect, the refractive index of the binding agent must be less than that of the hollow glass balls.
As standard materials in the formulation of protective and gloss lacquers di(meth)acrylate-based synthetics are used with either no or only very little filling. Due to their lack of fillers or the only very low filler loading tooth lacquers, compared with flow and filling composites, tooth lacquers have even lower viscosities. They are as a rule thin- to medium-thin-flowing and are removed from a bottle with a microapplication instrument, for example with a small brush or small paintbrush and then applied directly. Alternatively the lacquer system can be applied from a syringe by means of needles.
Medium-thin-flowing or also slow-flowing materials can be better controlled during application while thin-flowing systems allow a thinner application and can also flow into very small cavities. The application in just a thin coating is therefore important so that occlusion with the antagonistic tooth is not lastingly disrupted. Furthermore, the thinner the application of the transparent lacquer, the more accurate is the impression given of the color of the lacquered tooth and thus the natural tooth situation.
However, as a result of the reduced amount of filler or the complete lack of this, the lacquer systems have lower resistance to abrasion and flexural strength than medium- to highly-filled materials.
If dental, radically curable compositions are polymerized, then the upper layer of the polymer often appears to be sticky. The surfaces have not totally cured. This phenomenon is triggered by the oxygen in the air, which is a triplet diradical, in that the oxygen molecules disrupt and inhibit the radical polymerization due to their radical nature. Such a moist-appearing layer must be removed with alcohol. If through the selection of the monomers and the selection and adjustment of the initiator system it is managed to prevent the formation of an oxygen inhibition layer, then this lacquer system will have better sheen characteristics and greater resistance to the influences of discoloring agents
Low-filled or unfilled dental lacquering systems are suitable for use as “liquid polishing materials”, “dental varnish”, “coating materials”, “protective lacquer”, “gloss lacquer” and so on, for a large number of intra- and extra-oral situations and for use in the dental laboratory. Particularly suitable is their clinical use for                coatings of the marginal areas of old and new, direct and indirect restorations in synthetic materials and synthetic material-reinforced glass ionomer cements,        restorations and temporary prostheses for improving aesthetics and simplifying finishing and polishing,        treatment of defects in the natural tooth structure.        
The requirements on protective and gloss lacquer materials are therefore extremely varied. To begin with, transparent compositions should remain on the tooth surface for a long time in order to achieve their purpose. In so doing, they should withstand regular mechanical attacks on them for example from oral hygiene measures such as tooth brushing.
Since the lacquer materials in the oral cavity are exposed to permanent and highly-intensive hydrolytic attack, for example from saliva, the polymers should have the lowest possible water absorption. During radical cross-linking of methacrylate/acrylate compositions a three-dimensionally linked network results. Because of the very low size of the water molecule water can diffuse into the mesh of the polymer, where it accumulates at certain points in the network and there forms hydrogen bonds or other weak polar bonds. The more polar components that are present in the polymer matrix, the easier it is for further water absorption to take place. As a result of water absorption the polymer expands and an increase in the intermolecular distances take place. This hygroscopic expansion can lead to a structural reorganization of the polymer chains. The water absorption takes place over a long period following curing, so that when there is excessive water absorption an expansion stress occurs and thus an “overflowing” of the material. In the process parts of the lacquer may come away from the tooth surface and the above-mentioned “islands” form which then become the starting point for new bacterial colonizations.
On top of this water molecules can attack sensitive structure elements of the polymer such as for example ester groups, hydrolytically splitting these. This deterioration can lead to complete disintegration of the network and thus to a complete loss of the product.
On the other hand, a certain polarity of the lacquer systems is indeed desirable, since the tooth structure is hydrophilic and polar compositions guarantee a good adaptation and wettability of the material to the tooth substrate.